Electric blasting cap



March 14, 1939. c. R. JOHNSON ET AL ELECTRIC BLASTING CAP Original Filed Jan. 4, 1935 K L 1 I g "11 E H 2 ED C C B B A A C har/es R. Johnson William fi/(z'rsf ATTORNEY.

INVENTORS Patented Mar. 14, 1939 ELECTRIC BLASTING CAP Charles R. Johnson, Glen Mills, Pa., and William E. Kirst, Woodbury, N. J., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application January 4, 1935, Serial No. 314 Renewed July 16, 1938 3 Claims.

The present invention relates to electric initiators and more particularly to electric blasting caps and to a'new method for sealing said caps.

The two types of electric initiators commonly used in commercial blasting work may be generally classed as electric squibs and electric blasting caps. Electric squibs contain an explosive charge be achieved by the use of a plug of a suitable giving a flame of considerable length and duration on firing and are adapted for use with defiagrating explosives such as black powder. Electric blasting caps, on the other hand,.contain one or more charges of a detonating explosive and are used in the firing of high explosives of the dynamite type. Both types of initiators are caused to function by the passage of an electric current through a highly resistant bridge wire embedded in the ignition charge. The bridge wire serves to connect the two leg wires carrying the electric current. In the case of electric blasting caps particularly, these leg wires are commonly held in spaced relationship with one another by a bridge plug bf a material of relatively low fusion point cast around the wires. This plug is usually formed of sulfur and is sealed into the detonator by means of a superimposed waterproofing composition and a sulfur seal.

Many attempts have been made to, devise an improved means for waterproofing electric initiators, largely because of certain undesirable consequences of the use of the sealing materials ordinarily employed for this purpose. The low temperature at which this material becomes semifiuid is of advantage in some cases but is frequently the cause of deterioration of the detonator under conditions of warm storage.

Water resistance is, therefore, variable, depending to an unfortunate extent upon the storage history of the cap. To minimize this effect, it is the practice to use a relatively large amount of the sealing material. Consequently, a longer shell is required than would otherwise be necessary. On the other hand, at low temperatures, the water'- proofing material contracts and is brittle, thus tending to show poor adhesion to the cap shell.

This frequently results in the ingress of water or moist air. In addition, the procedure heretofore used involves a large amount of hand labor, since it does not readily lend itself to a mechanical assembly.

An object of our invention is a new and improved electric initiator. A further object is an electric blasting cap of improved water resistance. A still further object is an electric blasting cap 5 having animproved mechanically-assembled plug many ways.

may be bent inward toward the axis of the shell and seal which permits the use of a shell somewhat shorter than prior art practice. An additional object is anew and improved method for producing electric initiators of the type described. Other objects will become apparent as the inven- 5 tion is describedfully hereinafter.

We have found that the foregoing objects may metal, such as copper, brass, or aluminum. In any case, the plug is designed to fit snugly into the detonator shell. The detonator is then sealed by a closure means extending over the clearance space between the plug and the shell wall and mechanically engaging said plug and shell wall.

In order to describe our invention more clearly, we shall refer by way of illustration to the attached diagrammatic representations of particular embodiments. Figures 1 to 5 inclusive are longitudinal sections of electric blasting caps, the same letter referring to similar parts in the respective examples. Figures 6 and 7 are longitudi- \4 nal sections of the sealing plug.

A represents the shell of an electric blasting cap containing a base charge B, a primary charge 0,

- and a loose ignition charge D surrounding a bridge wire E between the two leading wires F. The top of the cap is closed by a plug G through which the leading wires F pass in the channels H provided for that purpose. The plug may be provided with a rim or flange I, which in Figure 5 is formed by an annular groove upon the upper surface of the plug. The cap is sealed by a closure means extending over the clearance space J between the plug and the shell wall and mechanically engagingthe plug and the shell wall. This closure means may comprise the rim of the shell wall (as in Figure 1) an extension of the upper portion of the plug (as in Figure 2) both the rim of the shell wall and an extension of the upper portion of the plug (as in Figure 3) or a separate annular device, such as a ferrule (as in Figures 4 and 5). The sealing of the cap may be accomplished in For example, the rim of the shell and crimped against an extension of the upper portion of the plug (as in Figure 1). Again, the extension of the upper portion of the plug or a lug thereon may be bent outward away from the axis of the shell and be crimped against the shell wall (as in Figure 2). By another arrangement both the rim of the shell and an extension of the upper portion of the shell may be bent so as to interlock (as in Figure 3), or the rim of the shell and an. extension of the upper portion of the plug may be workable material, for example of copper or the same material as the cap shell, is provided with two properly spaced holes extending throughout its length, and with a rim around its upper face. Such a plug is represented in vertical section by Figure 6. After the insulated wires have been threaded through the holes in the plug, it is passed through a suitable die, which compresses the material about the insulated wires, therebyv firmly and hermetically sealing them to the plu and at the same time shaping the latter so that it fits snugly into the mouth of the shell. Figure '7 represents the plug after such a treatment. After the bridge wires have been attached, the plug is placed in the loaded cap shell and the engagement between the rim of the shell and the extension on the upper portion of the plug is brought about in any desired manner, such as those described in the foregoing.

The advantages of such a closure for an electric blasting cap are numerous. The length of the shell can be reduced by one-third or more. The seal is so firm that the resultant detonator may be subjected to great water pressure without endangering the contents. The material from which the plug is made necessitates no special conditions of storage. In addition, the detonator may be assembled automatically and mechanically, if desired.

Having now described our invention in detail, it is obvious that many variations may be made without departing from the spirit of the invention. We, therefore, do not intend to be limited except as indicated in the following patent claims.

We claim:

1. In an electric blasting cap, a metal shell, a metal bridge plug adapted to close the open end thereof, leg wiresextending separately through the bridge plug in dielectric and water-impervious relation therewith, and a closure means comprising the upper portion of the shell wall extending over the clearance space between the plug and said shell wall, and mechanically compressing the shell wall about the plug to seal the cap against moisture penetration. 1

2. In an electric blasting cap, a metal shell, a metal bridge plug adapted to close the open end thereof, leg wires extending separately through the bridge plug in dielectric and water-impervious relation therewith, and a closure means comprising an annular metal device engaging the plug and the shell wall and extending over the clearance space between the plug and the shell wall, and mechanically compressing the shell wall about the plug to seal the cap against inner of said surfaces engaging the top of the metal plug.

CHARLES R. JOHNSON.

WILLIAM E. KIRST. 

